1. Field of the Invention
This invention relates to an injection molding machine comprising a horizontally movable clamping unit, a horizontally injecting injecting unit and a stationary mold carrier adapted to selectively carry a center gate injection mold section having a center gate and defining a central mold cavity and an off-center gate injection mold section having an off-center gate and defining a central mold cavity. The injecting unit is horizontally shiftable between two mutually opposite extreme injecting positions transversely to the injection axis of said injecting unit and is operable in said injecting positions to inject plasticized plastic material into the parallel gates of said two molds. Said mold carrier is formed with a cylinder-receiving opening, which is adapted to receive the plasticizing cylinder of the injecting unit and is approximately symmetrical to the horizontal plane of symmetry of the mold carrier and elongated in the direction in which said injecting unit is horizontally shiftable. The machine also comprises two advance-retract cylinders for moving said plasticizing cylinder into and out of engagement with said mold section, and also comprises guiding means for guiding the piston rods of the advance-retract cylinders. Said piston rods extend as far as to said mold carrier and during the shifting of the injecting unit are horizontally guided, and said piston rods are disposed on the left and right, respectively, of said cylinder-receiving opening. Means are provided for locking said piston rods in their axial position. The invention relates also to a stationary mold carrier for use in such machine.
2. Description of the Prior Art
An injection molding machine of that kind has been disclosed in U.S. Pat. No. 4,865,534 and permits a selective use of off-center gate injection molds which have a central mold cavity and an off-center gate, which may be disposed at different locations. In that machine the plasticized plastic is injected into the mold cavity through a gate which is approximately parallel to the direction of the mold-closing movement of the clamping unit. That gate opens into the mold cavity near the edge thereof. Such a gate may be described as a parallel gate to distinguish it from a gate which extends at right angles to the direction of said mold-closing movement and opens into the mold cavity in the parting plane of the injection mold. In the known injection molding machine the moldside ends of the piston rods of the advance-retract cylinders are guided in T-section grooves of the mold carrier as the injection unit is shifted between different injecting positions. During the injection of the plastic material, the piston rods are subjected to a force which opposes the pressure under which the plasticizing cylinder is forced against the mold and said force forces the piston rod ends against vertical side faces of the grooves of the mold carrier to hold the piston rods in position. As it is necessary to provide the grooves above and below the cylinder-receiving opening, the known mold carrier can be used only in injection molding machines in which the piston rods are symmetrically disposed with respect to the injection axis and arranged in a plane that includes an angle with the horizontal.